1. Field of the Invention
This invention relates to random access memories (RAMs), and more particularly to a high density layout for fast, stable RAMs.
2. Description of the Related Art
Heretofore, RAMs have employed conductive polysilicon material for the row word line. The distributed resistance of the poly was typically 25-100 ohms per square, which is high when compared to aluminum metal at less than 0.1 ohms per square. The relatively high poly resistance resulted in a high RC time constant and corresponding delays in the word line access cycle. The poly word line approach places design limitations on the number of memory cells permitted in a single word line.
The blanket silicon oxide layers used for insulation in integrated circuits require etched contact apertures. The etching material developes a slope along the aperture edge, which produces a lateral expansion of the aperture area. For apertures that penetrate several layers of insulative oxide, this expansion is cumulative and results in a high volume wide-deep aperture that is filled with metal or conductive poly. These large apertures take up cell space and reduce the insulation between the contact material in the aperture and the adjacent active elements. Narrow-deep apertures frequently have unfavorable aspect ratios (diameter over depth), and are difficult to fill during metal deposition.